JP2005169663A - Inkjet recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inkjet recording medium capable of recording on both surfaces, which has appearance similar to a perfecting printing sheet conventionally used for offset printing or gravure printing and different in front and rear surfaces in surface texture, exhibits excellent bookbinding properties preventing a coating layer from being cracked or peeled off even by being bent and can be suitably used for the front cover of a book-like printed matter such as a catalog or a book as a substitute for the perfecting printing sheet. <P>SOLUTION: A cast coating layer gloss-finished by a casting method is formed on one surface of an original paper sheet, and an ink receiving layer is formed on the other surface of the original paper sheet by laminating successively an undercoating layer containing alkaline earth metal salts and an upper-coating layer containing inorganic ultrafine particles. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an inkjet recording medium capable of double-sided recording. Specifically, the invention has a high glossy surface with a cast gloss finish, and a recording surface similar to a printing paper such as a coated paper or art paper, and is folded. The present invention also relates to an ink jet recording medium that has good bookbinding workability without cracking or peeling off the coating layer, and can be suitably used for a cover of a book-like printed material such as a catalog or a book.

The ink jet recording method is a printing method in which an image is recorded by ejecting ink droplets from a recording head and attaching the ink to a recording medium such as paper. As the ink jet recording medium, plain paper represented by PPC paper can be used. However, in the case of full-color applications such as photographs and high-definition illustrations that require high-definition printing, a higher level of ink absorbability can be obtained. It is necessary to use the provided recording medium. As such a recording medium, there is an ink jet recording medium having a configuration in which an ink receiving layer mainly composed of a pigment such as silica is coated on a substrate, which is called a coating type. This ink jet recording medium exhibits high ink absorbency due to the gaps between adjacent pigments in the ink receiving layer and the pores of the pigment particles themselves, and the ink drying speed is high, the image density is high, and there is little blurring. It has ideal ink jet characteristics. There has also been proposed an ink jet recording medium in which such a so-called porous (void) type ink receiving layer is coated on both surfaces of a base material, and double-sided recording is possible (for example, Patent Documents 1 to 3). reference).
JP-A-9-286166 JP 2001-39011 A JP 2002-103793 A

  With the spread of PCs and networks, advances in digital processing technology, etc., the applications of the ink jet recording method are rapidly expanding, and the transition from the conventional printing method to the ink jet recording method is progressing. For example, in the small lot printing and short run printing markets, attempts have been made to print booklets such as product catalogs and various pamphlets that have been conventionally performed by the offset printing method and the gravure printing method by the ink jet recording method. Also, in the publishing industry, on-demand printing services that are ready to be printed and bound from the first volume in response to user orders and immediately sent out are becoming full-fledged. As an ink jet recording method, attention has been paid.

  As described above, when printing on booklet-like printed materials such as catalogs and books is replaced by the inkjet recording method, the inkjet recording medium is comprehensively grasped from the color tone and glossiness as well as high recording quality. The appearance of the inkjet recording medium is required to be similar to the appearance of printing paper such as coated paper or art paper that has been conventionally used. Recently, in particular, product catalogs and other products that use double-sided printing paper (double-sided printing paper) with different surface textures on the front and back are used as the front cover, and the texture is modulated on the back cover side and the facing page side. Accordingly, there is an increasing need for an ink jet recording medium having an appearance similar to that of such double-sided printing paper.

  However, the conventional inkjet recording mediums compatible with double-sided printing as disclosed in Patent Documents 1 to 3 are significantly different in appearance from the above-described double-sided printing papers having different surface textures on the front and back sides. It could not be used as a substitute for. In addition, since the ink jet recording medium is used for a booklet-like printed material, when the booklet is formed into a booklet after printing, the ink receiving layer is not cracked or peeled off at the fold portion. Although it is necessary to have bookbinding processing suitability (folding resistance), there is room for improvement in the conventional ink jet recording medium in this respect as well, and it is insufficient as a sheet used for booklet-like bookbinding.

  Accordingly, an object of the present invention is to have a similar appearance to a double-sided printing paper having a different surface texture on the front and back, which has been used in conventional offset printing and gravure printing, and the coating layer is cracked even when folded. Ink jet recording medium capable of double-sided recording, which has good bookbinding processing suitability without being peeled off or peeled off, and can be suitably used as a cover of a book-like printed matter such as a catalog or book as an alternative to the double-sided printing paper Is to provide.

  In order to achieve the above object, the inkjet recording medium of the present invention has a cast coat layer gloss-finished by a casting method on one side of a base paper, and an alkaline earth metal salt on the other side of the base paper. It has an ink receiving layer formed by sequentially laminating an undercoat layer containing, and an overcoat layer containing inorganic ultrafine particles.

  The ink jet recording medium of the present invention is capable of double-sided recording and has excellent bookbinding processability, so that the coating layer (cast coat layer and ink receiving layer) may crack or peel off even when folded. Therefore, it is suitable as a paper used for booklet-like printed materials such as catalogs, pamphlets and books. In particular, since the ink jet recording medium has different surface textures on the front and back sides, it is most suitable as a cover for, for example, producing booklet printed materials having different textures on the back cover side and the spread page side.

  Hereinafter, the ink jet recording medium of the present invention will be described in detail. The inkjet recording medium of the present invention is an inkjet recording medium capable of so-called coating-type double-sided recording, and is coated on a base paper, a cast coat layer coated on one side of the base paper, and on the other side. And an ink receiving layer.

  As the base paper, paper mainly composed of cellulose pulp is preferably used. As raw materials for cellulose pulp, usually wood materials such as conifers and hardwoods are used, but besides these, grass fibers such as straw, esparto, bagasse and kenaf, and bast fibers such as hemp, cocoon, husk and trichome Or cotton can be used. In this invention, 1 type of these pulp raw materials may be used independently, and 2 or more types may be used together. The pulping method is not particularly limited, and a known pulping method such as kraft pulp, sulfite pulp, groundwood pulp, semi-chemical pulp, chemi-ground pulp, refiner ground pulp or the like can be used. Recycled pulp (deinked pulp) made from waste office paper, old newspapers, old magazines and the like is also preferably used in the present invention. In addition, inorganic fibers such as glass fibers, or synthetic fibers such as polyester fibers and aramid fibers can be appropriately used as necessary.

  In the base paper, various auxiliaries usually used when paper is made can be blended. For example, inorganic fillers such as clay, talc, light calcium carbonate, calcined kaolin, aluminum oxide, aluminum hydroxide, titanium oxide as fillers to improve paper flexibility, surface smoothness, opacity, printability, etc. Can be used. Further, a rosin sizing agent can be used as an internal sizing agent, and in this case, a sulfuric acid band can be used in combination as a fixing agent. When a neutral sizing agent such as an alkyl ketene dimer or alkenyl succinic acid is used, a cationic fixing aid such as cationic starch can be used in combination. Furthermore, if necessary, a polyacrylamide polymer, starch, or the like can be used as a paper strength enhancer.

If necessary, various pigment coating compositions may be preliminarily coated on one side or both sides of the base paper, and the coating amount in that case is 5 to 30 g / m in dry weight per side. ^{About 2} is desirable. If necessary, the base paper can be preliminarily subjected to a smoothing process such as super calendering, brushing, and cast finishing.

  The cast coat layer is a coating layer that has been gloss-finished by a casting method. In the casting method, a coating composition is applied onto the base paper, and the coating composition is pressed against a heated mirror surface of a cast drum in a wet or plastic state. Then, after drying, it is a glossy finishing method in which the mirror surface is copied by peeling. The casting method includes a direct method, a rewetting method, a coagulation method, and the like. Any method may be used in the present invention, but a cast coat layer having a gloss finish by the coagulation method is particularly preferable. The cast drum is usually a metal drum having a mirror-finished cylindrical outer surface.

  Any of the above casting methods is common in obtaining a high gloss coating layer surface by copying the cast drum surface, but until the coating composition coated on the base paper is pressed against the cast drum. Each process has the following characteristics.

In the case of the direct method: The coating composition applied to the base paper is pressed against the cast drum in a state where it is not dried at all.
In the case of the re-wetting method: The coating composition applied to the base paper is once dried or semi-dried, and after this state is made plastic by treatment with the re-wetting liquid, it is applied to the cast drum. Press contact.
In the case of the coagulation method: The coating composition applied to the base paper is brought into a non-flowable gel state by treatment with the coagulation liquid, and then pressed against the cast drum. The solidification method also includes a thermal solidification method in which the composition is irradiated with infrared rays to solidify the surface.

  The cast coat layer contains an inorganic pigment and an adhesive as essential components. Examples of inorganic pigments include silica, alumina, calcium carbonate, magnesium carbonate, barium sulfate, aluminum oxide, aluminum hydroxide, kaolin, talc, clay, calcined clay, satin white, aluminum silicate, magnesium silicate, titanium dioxide, and zinc oxide. And diatomaceous earth. In this invention, these 1 type may be used independently and 2 or more types may be used together.

  The inorganic pigment contained in the cast coat layer preferably has a primary particle size of 100 nm or less, and more preferably in the range of 5 to 50 nm. When the primary particle diameter of the inorganic pigment exceeds 100 nm, the transparency of the cast coat layer may be lowered, and as a result, it is difficult to obtain a sufficient surface gloss. Further, when the primary particle diameter of the inorganic pigment is less than 5 nm, the pores in the cast coat layer become too small, and the ink absorption rate may be lowered.

Examples of inorganic pigments that are particularly preferably used in the cast coat layer include synthetic silica such as amorphous silica, amorphous silica, and finely divided silica. By using synthetic silica, good ink absorptivity can be imparted to the cast coat layer. Among the synthetic silicas, those having a BET specific surface area in the range of 200 to 600 m ² / g are particularly preferable. If the BET specific surface area of the synthetic silica is less than 200 m ² / g, sufficient ink absorbability may not be obtained, and conversely if it exceeds 600 m ² / g, the continuous operability may be deteriorated. The proportion of synthetic silica in the total inorganic pigment in the cast coat layer is preferably 40 to 100% by weight.

  The content of the inorganic pigment in the cast coat layer is preferably in the range of 30 to 80% by weight with respect to the total solid content of the cast coat layer from the viewpoint of the balance between ink absorbency and coating film strength.

  The cast coat layer can contain an organic pigment (plastic pigment) that is deformed by heat in order to further increase the surface gloss. Especially as an organic pigment, what has a glass transition temperature in the range of 30-150 degreeC is preferable, and what is in the range of 50-130 degreeC is further more preferable. Organic pigments having a glass transition temperature in the above range are characterized in that they do not inhibit ink absorption and are easily glossy. Examples of such organic pigments include acrylic resins, polyolefin resins, microcapsules, urea resins, melamine resins, etc. Among them, acrylic resins are preferable, and acrylic resins containing methacrylic acid esters as structural units are particularly preferable. Resins are preferably used in the present invention because of their high gloss and ink absorbability. The primary particle diameter of the organic pigment is preferably 100 nm or less from the viewpoint of surface gloss.

  When an organic pigment is contained in the cast coat layer, the content is preferably 50% by weight or less based on the total solid content of the cast coat layer. If the content exceeds 50% by weight, ink absorbability may be hindered.

  On the other hand, examples of the adhesive contained in the cast coat layer include starches such as oxidized starch and esterified starch, cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose, polyvinyl alcohol and derivatives thereof, polyvinyl pyrrolidone, casein, gelatin, Examples include soy protein, styrene-acrylic resin and derivatives thereof, styrene-butadiene latex, acrylic emulsion, vinyl acetate emulsion, vinyl chloride emulsion, urethane emulsion, urea emulsion, alkyd emulsion, and derivatives thereof. In this invention, these 1 type may be used independently and 2 or more types may be used together.

  The content of the adhesive in the cast coat layer is preferably in the range of 10 to 100 parts by weight with respect to 100 parts by weight of the pigment (a total of the inorganic pigment and the organic pigment). When the content of the adhesive is less than 10 parts by weight, the binding of the pigment is insufficient, and the pigment is easily removed from the cast coat layer. On the other hand, when the content of the adhesive exceeds 100 parts by weight, the ink absorbability of the cast coat layer may be insufficient.

  Further, the cast coat layer can contain a release agent for the purpose of improving the release property from the cast mirror drum in the case of gloss finishing by the casting method. Examples of the mold release agent include fatty acids such as stearic acid, oleic acid, and palmitic acid, and salts thereof such as calcium, zinc, sodium, and ammonium, stearic acid amide, ethylene bis stearic acid amide, and methylene bis stearic acid. Amides such as amides, microcrystalline wax, paraffin wax, hydrocarbons such as polyethylene emulsion, higher alcohols such as cetyl alcohol and stearyl alcohol, fats and oils such as funnel oil and lecithin, various surfactants such as fluorine-containing surfactants Agents, fluorine polymers such as tetrafluoroethylene polymer and ethylene-tetrafluoroethylene polymer. Among these, stearic acid amide derivatives are particularly preferable, and specific examples thereof include stearic acid amide, methylol stearic acid amide, and ethylene bis stearic acid amide.

  The content of the release agent in the cast coat layer is in the range of 0.1 to 10% by weight with respect to the total solid content of the cast coat layer from the viewpoint of achieving both good recordability and high continuous operability. Is preferable, and it is more preferable to be in the range of 0.5 to 5% by weight. If the content of the release agent is less than 0.1% by weight, the effect of improving the releasability is poor, and conversely if it exceeds 10% by weight, the ink-absorbing ink may be lowered, and the image quality may be deteriorated.

  In the cast coat layer, in addition to the above-mentioned pigment, adhesive, release agent, if necessary, dye fixing agent, pigment dispersant, antifoaming agent, antifoaming agent, release agent, foaming agent, penetrating agent, Coloring dyes, coloring pigments, fluorescent brighteners, ultraviolet absorbers, antioxidants, preservatives, antibacterial agents, water resistance agents, wet paper strength enhancers, dry paper strength enhancers and the like can be appropriately contained.

  The various components of the cast coat layer described above are usually prepared as an aqueous coating composition. The coating composition is formed on the base paper using a known coating machine such as a blade coater, an air knife coater, a roll coater, a brush coater, a kiss coater, a squeeze coater, a curtain coater, a bar coater, a gravure coater, or a comma coater. Coated. After coating, a cast coat layer is formed through the various cast processes described above.

  In addition, when performing the cast treatment by the rehumidification method, for example, ammonium salts, polyamide resins, phosphorus compounds such as hexametaphosphoric acid, amide compounds, fluorides, zinc sulfate, calcium formate, etc. may be added to the rewetting liquid. it can.

  In addition, when performing a cast treatment by a coagulation method, examples of the coagulant that can be added to the coagulation liquid include, for example, formic acid, acetic acid, citric acid, tartaric acid, lactic acid, hydrochloric acid, sulfuric acid, calcium, zinc, barium, lead , Various salts with magnesium, cadmium, aluminum and the like, potassium sulfate, potassium citrate, borax, boric acid and the like. In the present invention, it is particularly preferable to use formate.

  Next, the ink receiving layer coated on the surface opposite to the cast coat layer across the base paper will be described. The ink receiving layer includes, in order from the base paper side, an undercoat layer containing an alkaline earth metal salt and an overcoat layer containing inorganic ultrafine particles.

  Alkaline earth metal is a generic term for beryllium, calcium, magnesium, strontium, barium, and radium. Examples of the alkaline earth metal salt contained in the undercoat layer include carbonates, silicates, borates, hydrochlorides, sulfates, organic acid salts, etc., but the undercoat layer coating solution is water-based. Therefore, weak acid salts with low solubility are preferred. In particular, calcium carbonate and magnesium carbonate are preferably used in the present invention.

  The content of the alkaline earth metal salt is preferably in the range of 50 to 95% by weight with respect to the total solid content of the undercoat layer. If the content of the alkaline earth metal salt is less than 50% by weight of the total solid content, the ink absorbability may be lowered, and if it exceeds 95% by weight, the adhesion of the undercoat layer to the base paper is lowered, and delamination occurs. There is a risk of it happening.

  The undercoat layer preferably contains an adhesive having a mass ratio of 0.05 to 0.8 times that of the alkaline earth metal salt. More preferably, it is the range of 0.05 times or more and 0.4 times or less. When the adhesive is less than 0.05 by mass ratio with respect to the alkaline earth metal salt, the adhesive strength is insufficient, and peeling occurs between the base paper and the topcoat layer. Further, if the adhesive exceeds 0.8 times by mass with respect to the alkaline earth metal salt, the ink absorbability is lowered, which is not preferable.

  Examples of the adhesive contained in the undercoat layer include cellulose-based adhesives such as methyl cellulose, methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, and hydroxyethyl cellulose, starch and modified products thereof, gelatin and modified products thereof, casein, pullulan, Gum arabic and natural polymer resins such as albumin or derivatives thereof, polyvinyl alcohol and modified products thereof, styrene-butadiene copolymer, styrene-acrylic copolymer, methyl methacrylate-butadiene copolymer, ethylene-vinyl acetate copolymer Latex and emulsions such as polymers, vinyl polymers such as polyacrylamide and polyvinylpyrrolidone, polyethyleneimine, polypropylene glycol, polyethylene glycol, and maleic anhydride or Copolymers. A copolymer-based emulsion is preferred.

  For the undercoat layer, in addition to alkaline earth metal salts and adhesives, a cationic dye fixing agent, a pigment dispersant, a thickener, a fluidity improver, a viscosity stabilizer, a pH adjuster, a surface active agent, if necessary. Agent, antifoaming agent, antifoaming agent, release agent, foaming agent, penetrating agent, coloring dye, coloring pigment, white inorganic pigment, white organic pigment, fluorescent whitening agent, ultraviolet absorber, antioxidant, leveling agent, Preservatives, antibacterial agents, water resistance agents, wet paper strength enhancers, dry paper strength enhancers and the like can be appropriately contained.

  On the other hand, the overcoat layer laminated on the undercoat layer contains inorganic ultrafine particles as an essential component. The inorganic ultrafine particles mean inorganic fine particles having a primary particle diameter of 100 nm or less and a secondary particle diameter of 400 nm or less. For example, JP-A-1-97678, 2-275510, 3-281383, 3-285814, 3-285815, 4-92183, 4-267180 Pseudo boehmite sol which is an alumina hydrate disclosed in Japanese Patent Laid-Open Nos. 4-27517, etc., JP-A-60-219083, 61-19389, 61-188183, 63- Colloidal silica as described in JP 178074 A, JP 5-51470 A, etc., silica / alumina hybrid sol as described in JP 4-19037 A, JP 62-286787 A, etc. , Gas phase process silica as described in JP-A-10-119423 and JP-A-10-217601. Silica sol dispersed with a high-speed homogenizer, and other smectite clays such as hectite and montmorillonite (JP-A-7-81210), zirconia sol, chromia sol, yttria sol, ceria sol, iron oxide sol, zircon sol, antimony oxide sol, etc. Can be cited as representative.

  Among these inorganic ultrafine particles, gas phase method silica ultrafine particles and alumina compounds (alumina hydrate, aluminum oxide ultrafine particles) are particularly suitably used for the overcoat layer.

Silica fine particles are fine particles composed of 93% or more of SiO _2, about 5% or less of Al ₂ O _{3 and} about 5% or less of Na ₂ O on a dry basis. There is crystalline silica. As methods for producing amorphous silica fine particles, there are a liquid phase method, a pulverized solid phase method, a crystallization solid phase method, and a gas phase method. The liquid phase method is a fine particle production method in which a silicate compound or the like existing in a so-called liquid is precipitated in a solid state by a chemical change or a physical change. The pulverized solid phase method is a method for mechanically pulverizing silica solids, and the crystallization solid phase method is a method for producing fine particles using melting, solid phase transition, or the like. The gas phase method is a method for producing fine particles by thermal decomposition of vapor of a volatile metal compound, heating and evaporation of raw materials, cooling of a generated gas phase species, and condensation.

  Among the above, silica fine particles that can be suitably used for the topcoat layer are amorphous silica fine particles synthesized by a gas phase method. Among these, ultrafine silica having an average primary particle diameter of 3 to 50 nm is preferable. A particularly preferable primary particle diameter is 5 to 30 nm. In addition, the secondary particle diameter in which these are connected is preferably 10 to 200 nm, but it is not certain what form the coating layer will take, and the size of the secondary particles is not a problem. Absent. Aerosil (Tegusa Co., Ltd.) is a product that is commercially available as amorphous silica fine particles synthesized by this vapor phase method.

  The vapor phase silica used in the present invention is obtained by adding silica fine particles having the above primary particle size to water and dispersing with a high-speed homogenizer or the like, and dispersing the average secondary particle size to 200 nm or less, preferably 100 nm or less. It is.

The alumina hydrate used in the present invention can be represented by the general formula Al ₂ O ₃ .nH ₂ O. Alumina hydrates are classified into dipsite, vialite, norstrandite, boehmite, boehmite gel (pseudoboehmite), diaspore, amorphous amorphous, etc., depending on the composition and crystal form. In particular, in the above formula, when the value of n is 1, it represents an alumina hydrate having a boehmite structure, and when n is more than 1 and less than 3, it represents an alumina hydrate having a pseudo boehmite structure, where n is 3 or more represents an amorphous alumina hydrate. In particular, the preferred alumina hydrate for the present invention is an alumina hydrate having a pseudo boehmite structure in which at least n is more than 1 and less than 3.

  The shape of the alumina hydrate may be any of a flat plate shape, a fiber shape, a needle shape, a spherical shape, a rod shape and the like, and a preferable shape is a flat plate shape from the viewpoint of ink absorbability. The plate-like alumina hydrate has an average aspect ratio of 3 to 8, and preferably an average aspect ratio of 3 to 6. The aspect ratio is expressed as the ratio of the “diameter” to the “thickness” of the particles. Here, the diameter of the particle represents the diameter of a circle equal to the projected area of the particle when the alumina hydrate is observed with an electron microscope. When the aspect ratio is smaller than the above range, the pore diameter distribution of the overcoat layer becomes narrow, and the ink absorbability is lowered. On the other hand, when the aspect ratio exceeds the above range, it becomes difficult to produce alumina hydrate by aligning the particles.

  The alumina hydrate used in the present invention can be produced by a known method such as hydrolysis of aluminum alkoxide such as aluminum isopropoxide, neutralization of aluminum salt with alkali, hydrolysis of aluminate. The physical properties of alumina hydrate, such as particle size, pore size, pore volume, and specific surface area, should be controlled by conditions such as precipitation temperature, aging temperature, aging time, solution pH, solution concentration, and coexisting compounds. Can do.

  As methods for obtaining alumina hydrates from alkoxides, JP-A-57-88074, JP-A-62-256321, JP-A-4-275717, JP-A-6-64918, JP-A-7-10535, JP-A-7-267633, US Pat. No. 2,656,321 and the like disclose a method for hydrolyzing aluminum alkoxide. These aluminum alkoxides include isopropoxide, 2-butoxide and the like.

  JP-A Nos. 54-116398, 55-23034, 55-27824, and 56-120508 disclose a method using an inorganic salt of aluminum or a hydrate thereof as a raw material. It is disclosed. Examples of the raw material include inorganic salts such as aluminum chloride, aluminum nitrate, aluminum sulfate, polyaluminum chloride, ammonium alum, sodium aluminate, potassium aluminate, aluminum hydroxide, and hydrates thereof.

  Further, as another method, a method described in JP-A-56-120508, in which the pH is alternately changed from the acidic side to the basic side to grow alumina hydrate crystals, Japanese Patent Publication No. 4-33728. There is also a method of rehydrating alumina by mixing alumina hydrate obtained from an inorganic salt of aluminum described in the publication and alumina obtained by the Bayer method.

  Commercially available alumina hydrate can be used for the topcoat layer. One example is given below, but the present invention is not limited thereto. For example, as alumina hydrate, Cataloid AS-1, Cataloid AS-2, Cataloid AS-3 (above, manufactured by Catalytic Chemical Industry) Alumina sol 100, Alumina sol 200, Alumina sol 520 (above, manufactured by Nissan Chemical Industries), M- 200 (above, manufactured by Mizusawa Chemical Industry Co., Ltd.), aluminum sol 10, aluminum sol 20, aluminum sol 132, aluminum sol 132S, aluminum sol SH5, aluminum sol CSA55, aluminum sol SV102, aluminum sol SB52 (manufactured by Kawaken Fine Chemical).

  As the aluminum oxide ultrafine particles, γ-type aluminum oxide fine particles which are γ-type crystals can be suitably used for the overcoat layer. If the γ-type crystal is classified crystallographically, it can be further divided into a γ group and a δ group. Fine particles having a δ group crystal form are preferred.

  The alumina of the γ-type crystal fine particles can reduce the average particle diameter of the primary particles to about 10 nm, but generally the primary particles form a secondary aggregated form (hereinafter referred to as secondary particles). The particle diameter increases from several thousand to several tens of thousands nm. When such γ-type alumina crystal fine particles having a large particle diameter are used, the printability and absorbency are good, but the transparency is insufficient and coating film defects tend to occur. The average particle diameter of the primary particles is preferably less than 80 nm. When secondary particles composed of primary particles of 80 nm or more are used, brittleness increases and coating film defects are very likely to occur.

  In order to obtain a γ-type alumina crystal fine particle sol, the average particle size is usually obtained by crushing γ-type alumina crystals, which are secondary particles of several thousand to several tens of thousands nm, by a grinding means such as a bead mill, an ultrasonic homogenizer, or a high-pressure homogenizer. Is pulverized until it becomes ultrafine particles of 200 nm or less, preferably 100 nm or less. When the average particle diameter exceeds 200 nm, the ink absorbability increases, but the coating becomes brittle and the transparency is lowered. As the pulverization means, a method using an ultrasonic homogenizer or a high-pressure homogenizer is preferable, and in other pulverization methods such as a bead mill, the γ-type alumina crystal is a hard crystal, so that foreign matters are easily mixed from the pulverization container, and transparency. Cause deterioration and defects. γ-type alumina crystal fine particles have excellent ink absorbability, good print quality such as dryness and ink fixability, and ultra-fine particles make ink jet recording excellent in transparency even when included in the overcoat layer at a high ratio. A medium can be obtained.

  The γ-type alumina crystal fine particles are commercially available as aluminum oxide C belonging to the δ group (manufactured by Nippon Aerosil Co., Ltd.), AKP-G015 belonging to the γ group (manufactured by Sumitomo Chemical Co., Ltd.), and the like.

  The content of the inorganic ultrafine particles used in the topcoat layer is preferably in the range of 40 to 90% by weight with respect to the total solid content of the topcoat layer. If the content of inorganic ultrafine particles is less than 40% by weight, there is a risk that ink absorbability is insufficient and satisfactory in image quality may not be obtained, and if it exceeds 90% by weight, problems such as powder falling due to insufficient coating strength. May be incurred.

  The topcoat layer can contain a water-soluble or water-insoluble polymer compound having an affinity for ink as an adhesive for inorganic ultrafine particles. Examples of the water-soluble polymer compound include cellulose-based adhesives such as methylcellulose, methylhydroxyethylcellulose, methylhydroxypropylcellulose, and hydroxyethylcellulose, starch and modified products thereof, gelatin and modified products thereof, casein, pullulan, gum arabic, And natural polymer resins such as albumin or derivatives thereof, polyvinyl alcohol and modified products thereof, styrene-butadiene copolymer, styrene-acrylic copolymer, methyl methacrylate-butadiene copolymer, ethylene-vinyl acetate copolymer, etc. Latex, emulsions, vinyl polymers such as polyacrylamide and polyvinylpyrrolidone, polyethyleneimine, polypropylene glycol, polyethylene glycol, and maleic anhydride Like the union or the like. Polyvinyl alcohol is preferable.

  As the water-insoluble polymer compound, a water-insoluble adhesive that dissolves in alcohols such as ethanol and 2-propanol or a mixed solvent of these alcohols and water can stabilize the dispersion of aluminum oxide. Particularly preferred. Examples of such water-insoluble adhesives include acetal resins such as vinylpyrrolidone / vinyl acetate copolymer, polyvinyl butyral, and polyvinyl formal, and in particular, the degree of acetalization is in the range of 5 mol% or more and 20 mol% or less. This acetal resin is particularly preferred because it can contain some water and facilitate the dispersion of the inorganic ultrafine particles.

  These polymer compounds may be used alone or in combination, and the content thereof is preferably 2 to 50% by weight, more preferably 5 to 30% by weight, based on the inorganic ultrafine particles. If the content of the polymer compound is less than 2% by weight with respect to the inorganic ultrafine particles, the coating strength is insufficient, and conversely if it exceeds 50% by weight, the ink absorbability may be lowered.

  In addition to the inorganic ultrafine particles and polymer compound (adhesive), the topcoat layer may contain a pigment dispersant, a thickener, a fluidity improver, a viscosity stabilizer, a pH adjuster, a surfactant, Foaming agent, antifoaming agent, mold release agent, foaming agent, penetrating agent, coloring dye, coloring pigment, white inorganic pigment, white organic pigment, fluorescent whitening agent, UV absorber, antioxidant, leveling agent, preservative, Antibiotic agents, water resistance agents, wet paper strength enhancers, dry paper strength enhancers, and the like can be appropriately contained.

  The ink receiving layer composed of the undercoat layer and the overcoat layer as described above is prepared by coating a separately prepared undercoat layer coating solution on the base paper and drying to form the undercoat layer. It can form by applying the coating liquid for topcoat layers prepared separately, and drying and forming a topcoat layer. The coating of coating liquid is air knife coater, curtain coater, slide lip coater, die coater, blade coater, gate roll coater, bar coater, comma coater, rod coater, roll coater, bill blade coater, short dwell blade coater, It can carry out as usual using various coating apparatuses such as a size press. If necessary, the undercoating layer or the overcoating layer (the entire ink receiving layer) can be calendered to enhance smoothness and gloss. Examples of the calendar processing device at that time include a gloss calendar, a super calendar, and a soft calendar. In particular, a thermal calendar process that performs a smoothing process while applying heat is preferably used.

  In addition, it is preferable that the said coating liquid for topcoat layers exists in an acidic region, Preferably it is pH 5 or less, More preferably, it is pH 4 or less. If the coating liquid for the topcoat layer is in the alkaline region, the ink absorbability may not be sufficiently obtained, and if the pH of the coating solution exceeds 5 even in the acidic region, the ink absorbability decreases. It is because it is in a tendency. As described above, the reason why the ink absorbency of the recording medium is increased by lowering the pH of the overcoat layer coating solution is not clear, but the pH is in the acidic range on the undercoat layer formed on the base paper. When the topcoat layer coating solution is applied, the alkaline earth metal salt in the undercoat layer causes the pH of the topcoat layer coating solution to gel, resulting in inorganic fine particles in the topcoat layer coating solution. This is presumably due to the prevention of penetration into the undercoat layer.

  In the ink jet recording medium of the present invention, an ink receiving layer was formed by sequentially laminating an undercoat layer and an overcoat layer on one side of a base paper, and the ink receiving layer was calendered as necessary. Thereafter, it is preferably produced by forming a cast coat layer on the other side of the base paper. This is because if the cast coat layer is formed first, the cast coat layer may be damaged in the subsequent ink receiving layer forming step or the like.

  The thickness of each coating layer is preferably in the range of 10 to 30 μm for both the cast coat layer and the ink receiving layer from the viewpoint of the balance between ink absorbability and bookbinding processability (folding resistance). If the thickness of each coating layer is less than 10 μm, the ink absorbability may be insufficient and the recording quality may be unsatisfactory. On the other hand, if the thickness of each coating layer exceeds 30 μm, after printing, when the inkjet recording medium is bound into a booklet, there is a possibility that the coating layer may be cracked or scratched at the crease part, which is severe In some cases, the coating layer may have large cracks, or a part of the coating layer may be peeled off.

  Also, even if the thickness of each coating layer is in the above range, the base paper is thick so that it is difficult to bend when the entire inkjet recording medium is thick. There is a risk of serious damage to the center. Therefore, from the viewpoint of minimizing damage to the coating layer when bent, the total thickness of the ink jet recording medium is preferably 170 μm or less, and more preferably 150 μm or less. When used for the cover of a booklet-like printed material, the thickness of the entire inkjet recording medium is preferably in the range of 70 to 150 μm.

  The ink jet recording medium of the present invention can be suitably used for ink jet recording, and the ink used in that case is not particularly limited. In general, water-based pigment inks have difficulty in color reproducibility and are not suitable for applications in which high image quality is pursued. However, according to the inkjet recording medium of the present invention, high-quality inkjet images can be obtained even with water-based pigment inks. Is obtained. A pigment ink for inkjet recording is usually prepared by containing a pigment-based color material in an aqueous medium composed of water, various organic solvents, a surfactant and the like. Examples of pigment-based color materials include titanium oxide and iron oxide, inorganic pigments such as carbon black produced by known methods such as contact method, furnace method, thermal method; azo pigments (azo lakes, insoluble azo pigments, condensed azo pigments) , Chelate azo pigments, etc.), polycyclic pigments (eg, phthalocyanine pigments, perylene pigments, perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, quinofullerone pigments, etc.), dye chelates (eg, Organic dyes such as nitro pigments, nitroso pigments, aniline black, and the like. The content of the pigment-based color material is usually about 0.5 to 30% by weight with respect to the total weight of the ink. When color ink jet recording is performed, three or more subtractive primary inks of yellow (Y), magenta (M), and cyan (C), or black (K) and other colors are added to four or more colors. Ink is used.

  Hereinafter, the present invention will be described more specifically with reference to examples of the present invention and test examples showing the effects of the present invention. However, the present invention is not limited to the examples.

<Making paper>
A 1: 1 mixture of hardwood bleached kraft pulp (LBKP, whiteness 90%) and softwood bleached sulfite pulp (NBSP, whiteness 90%) was beaten to 300 ml with Canadian Standard Freeness to prepare a pulp slurry. As a sizing agent, 0.5% by weight of alkyl ketene dimer to pulp, 1.0% by weight of polyacrylamide to pulp as a paper strength enhancer, 2.0% by weight of cationized starch to pulp, polyamide epichlorohydrin The resin was added at 0.5% by weight to the pulp and diluted with water to give a 1% slurry. After that, the stock slurry is formed into a paper web with a long paper machine, subjected to three stages of wet pressing with a wet part, then processed with a smoothing roll, and then subjected to two-stage tension pressing in the middle of the subsequent drying part. It was. Thereafter, a size press solution of 5% by weight of carboxy-modified polyvinyl alcohol is pressed at a size of 10 g / m ^{2 in} the course of drying, and dried so that the final base paper moisture is 8% by weight with absolutely dry moisture. The base paper was processed to a basis weight of 72 g / m ² (thickness 80 μm).

<Preparation of coating solution for undercoat layer>
100 parts by weight of light calcium carbonate (Tama Pearl 222H: manufactured by Okutama Kogyo Co., Ltd.) as an alkaline earth metal salt, and 20% by weight of solid content of styrene-butadiene copolymer latex (Luckster DS226, manufactured by Dainippon Ink Co., Ltd.) as an adhesive Part of the mixture was mixed to prepare an undercoat layer coating solution having a solid content of 45%.

<Preparation of coating liquid for topcoat layer>
Ion-exchanged water 1200 g and isopropyl alcohol 900 g were charged into a 3 L reactor and heated to 75 ° C. 408 g of aluminum isopropoxide was added, and hydrolysis was performed at 75 g for 24 hours and then at 95 ° C. for 10 hours. After hydrolysis, 24 g of acetic acid was added and stirred at 95 ° C. for 48 hours. Next, it was concentrated so that the solid content concentration was 15% by weight, and a dispersion of white ultrafine alumina hydrate was obtained. When this sol was dried at room temperature and measured for X-ray diffraction, it showed a pseudo-boehmite structure. Further, when the average particle diameter was measured with a transmission electron microscope, it was 30 nm and was a flat ultrafine particulate alumina hydrate having an aspect ratio of 6.0. Further, when the average pore radius, pore volume and BET specific surface area were measured by the nitrogen adsorption / desorption method, they were 7.1 nm, 0.65 ml / g and 200 m ² / g, respectively.

  15 parts by weight of a 10% by weight aqueous solution of polyvinyl alcohol (PVA105: manufactured by Kuraray Co., Ltd.) is used with respect to 100 parts by weight of the alumina hydrate dispersion using the above dispersion of 15% by weight of ultrafine alumina hydrate. Mixed. After mixing, the mixture was homogenously dispersed in a homomixer at 10,000 rpm for 10 minutes. The dispersion was concentrated by an evaporator so as to have a solid content of 11% to obtain a coating liquid for topcoat layer. The pH of this coating solution was 4.5.

<Preparation of coating solution for cast coat layer>
100 parts by weight of synthetic silica (Mizukasil P-87, manufactured by Mizusawa Chemical Co., Ltd.) having a BET specific surface area of 320 m ² / g as a pigment, and 10 parts by weight of styrene-butadiene latex (JSR-0617, manufactured by Nippon Synthetic Rubber Co., Ltd.) as an adhesive , 60 parts by weight of casein (Lactic casein, New Zealand) and 1.5 parts by weight of stearamide (Hydrin F-792, manufactured by Chukyo Yushi Co., Ltd.) as a release agent, respectively, with a solid content concentration of 30% An aqueous cast coat layer coating solution was prepared.

[Example 1]
On one surface of the base paper, the undercoat layer coating solution was applied and dried using a bar coater so that the thickness after drying was 15 μm, and an undercoat layer was applied. Next, on the undercoat layer, the above-mentioned overcoat layer coating solution was applied and dried using a bar coater so that the thickness after drying was 14 μm. The ink receiving layer thus formed was subjected to a linear pressure of 3 kg / cm and a processing speed of 30 cm / min. Using a desk calendar device manufactured by Kumagai Riki Kogyo. The calendering was performed under the processing conditions of a heating temperature of 50 ° C. Next, on the other side of the base paper, the above-mentioned coating liquid for cast coat layer was applied using a comma coater so that the thickness after drying was 30 μm, and then this was coagulated with a coagulating liquid. While the coating layer was in a wet state, the cast coating layer was applied by pressing and drying on a mirror-finished metal surface heated to 100 ° C. As the coagulation liquid, 5.0% by weight of calcium formate as a prepared coagulant and 1.0% by weight of a cationic polymer (Daifix YK-50, manufactured by Daiwa Chemical Co., Ltd.) as a water-resistant agent were contained. A thing was used. The ink jet recording medium produced as described above was used as a sample of Example 1.

[Examples 2 to 7]
In Example 1, an ink jet recording medium was prepared in the same manner as in Example 1 except that the thickness of each coating layer was appropriately changed as shown in Table 1 below. It was.

Example 8
In Example 1, an ink jet recording medium was prepared in the same manner as in Example 1 except that a base paper having a basis weight of 88 g / m ² (thickness 95 μm) made by the same method as in the above <Paper paper making> Was used as a sample of Example 8.

Example 9
In Example 1, an inkjet recording medium was prepared in the same manner as in Example 1 except that a base paper having a basis weight of 115 g / m ² (thickness 120 μm) made by the same method as in the above <Paper paper making> was used. Was used as a sample of Example 9.

[Comparative Example 1]
In Example 1, the coating solution for the topcoat layer was applied to each side of the base paper so that the thickness after drying was 28 μm on both sides, and the coating layer was applied by drying and applied. In the same manner as in Example 1, an ink jet recording medium (double-sided ink-jet coated paper) was prepared and used as a sample of Comparative Example 1.

[Comparative Example 2]
In Example 1, except that the cast coating layer coating liquid was applied to each side of the base paper so that the thickness after drying was 30 μm on both sides, and the cast coating layer was applied by drying. An ink jet recording medium (double-sided cast coated paper) was produced in the same manner as in Example 1, and this was used as a sample of Comparative Example 2.

[Test example]
About each said sample (inkjet recording medium), the white background gloss, appearance similarity, coloring property, and folding resistance were evaluated by the following methods, respectively. The evaluation results are shown in [Table 1] below.

(Evaluation method for white paper gloss)
According to TAPPI T480 om-90, the 75-degree glossiness of the surface of the coating layer of each sample (ink-receiving layer for Examples 1 to 7, coating layer on any side for Comparative Examples 1 and 2). When the measured value is 50% or more, A (glossiness equal to or better than art paper or coated paper), 40% or more and less than 50% is B (practical limit), and less than 40% C (unusable). The measured value of 75 degree gloss of the ISO standard paper type 1 (art paper) or type 3 (A2 gloss white coated paper) is about 60 ± 10%.

(Appearance similarity evaluation method)
Using X-Rite938 manufactured by Gretag Macbeth Co., under the conditions of a viewing angle of 2 degrees, a light source D50, and Black Backing, the coating layers of the above samples (in Examples 1 to 7, the ink receiving layer and Comparative Examples 1 and 2) Measure the CIELAB values (L ^* / a ^* / b ^* ) of the surface of the coating layer on any side, and these measured values and the ISO standard paper type 1 used for “Japan Color Color Reproduction Printing 2001” The color difference (ΔE ^* ) defined by JIS Z8730 is calculated from the CIELAB value (91/0 / -2) of (art paper) or type 3 (coated paper), and ΔE ^* is 4.0 or less. Appearance is similar to art paper and coated paper), ΔE ^* is more than 4.0 and less than 7.0 B (appearance limit), and ΔE ^* is 7.0 or more C (appearance with art paper and coated paper) Are not similar).

(Evaluation method of color development)
Each sample is set in an ink-jet printer PM-4000PX manufactured by Seiko Epson, and cyan (C), magenta (M), Y (yellow), and K (black) water-based pigment inks (PM) are applied to both surfaces of each sample. The color patches of each color were printed by ejecting the ink (standard-installed on -4000PX) at a duty of 100%. For these color patches, a reflection optical density (OD) was measured under the conditions of a viewing angle of 2 degrees, a light source D50, and no filter using a Spectrolino SPM-50 manufactured by Gretag Macbeth Co., Ltd., and evaluated according to the following evaluation criteria.
Evaluation criteria A: The sum of the ODs of the four colors of CMYK exceeds 7.5. High image density and good color development.
B: The total OD of the four colors is 7.5 to 6.0. There is no practical problem.
C: The total OD of the four colors is less than 6.0 (on average, less than OD1.5). Not practical.

(Folding resistance evaluation method)
The above samples are set on the PM-4000PX, and C, M, Y, and K water-based pigment inks (inks that are standardly installed on the PM-4000PX) are ejected on both sides of each sample at a duty of 100%. Each color patch was printed. Thus, after producing 10 printed matter for each sample, the cast coat layer becomes the front when folded using the saddle stitch bookbinding system (SPF-20 / FC-20) manufactured by Horizon Co., Ltd. In this manner, saddle stitch binding was performed, and a booklet-like printed material composed of 10 printed materials was produced for each sample. Further, as the blank, 10 sheets of commercially available printing paper (Pearl Coat (A2 gloss), manufactured by Mitsubishi Paper Industries, 128 g / m ² ) having almost the same amount as each of the above samples was saddle-stitched in the same manner as described above. The crease part in the spine of each booklet-like printed matter thus obtained is visually observed, and the coating layer has almost no cracks or scratches, and is comparable to the crease part of the blank. (Folding resistance of the coating layer is good. Bookbinding processability is good.) Although cracks and scratches are slightly conspicuous in the coating layer compared to the blank, B (no practical problem) C (practical limit) is the one in which cracks and scratches are conspicuous in the coating layer, and D (unpractical) is the one in which large cracks and scratches are exposed such that the base paper is exposed in the coating layer.

Claims (5)

  One side of the base paper has a cast coat layer that is gloss-finished by a casting method, and the other side of the base paper has an undercoat layer containing an alkaline earth metal salt and an overcoat layer containing inorganic ultrafine particles. An ink jet recording medium comprising an ink receiving layer that is sequentially laminated.   2. The ink jet recording medium according to claim 1, wherein the casting method is any one of a direct method, a rewetting method and a coagulation method.   3. The ink jet recording medium according to claim 1, wherein calcium carbonate and / or magnesium carbonate is used as the alkaline earth metal salt, and vapor phase silica and / or alumina compound is used as the inorganic ultrafine particles.   The inkjet recording medium according to any one of claims 1 to 3, wherein the cast coat layer and the ink receiving layer each have a thickness of 10 to 30 µm. The inkjet recording medium according to any one of claims 1 to 4, wherein the entire thickness of the inkjet recording medium is 170 µm or less.